What Causes Pus Filled Lumps On The Back?

hello i am writing for my grandpop...last year he was experiencing fatigue, stomach problems and dizziness/weakness. I noticed there was a large bump on his lower back with 2 openings so I attempted to squeeze it and a lot of smelly (foot/cheese-like) whitish/yellow semi-hard puss shot out. There was an enormous amount, but he did not complain of a lot of pain. Lately he has been experiencing the same symptoms as last year so I checked his back and the lump returned. this time there were 3 openings in an upside down triangular formation and the same puss came out when squeezed. Last year he felt better after the pus was removed...do you know what this is? thanks